This invention relates to a control system for an aircraft gas turbine engine and, more particularly, to a control system including a back-up control adapted to provide continued operation of an aircraft gas turbine engine in the event of malfunction or failure of the primary engine control system.
Full authority electronic control systems for gas turbine engines provide a number of significant advantages over conventional hydromechanical control systems. However, since full authority electronic control systems are only advancing to what might be termed a first-generation level, confidence in the reliability of such electronic controls has not attained the level generally associated with the hydromechanical systems in use for many years in highly refined configurations. Consequently, it is advisable to complement the electronic control system with a secondary or back-up hydromechanical control system which will ensure continued operation of the engine in the event the primary electrical control system exhibits a malfunction or a failure.
Many hydromechanical back-up control systems known in the prior art, such as that shown in U.S. Pat. No. 3,820,323, rely upon the pilot of the aircraft to assure that the gas turbine engines do not encounter engine stall or overtemperature during changes in the speed of the engine. In such a system the pilot has the responsibility of accelerating or decelerating the engine within certain rate limits in order to avoid the aforementioned engine stall and overtemperature. Specifically, the pilot, while increasing engine thrust or speed, must be cognizant of possible engine stall or overtemperature and must direct his attention at advancing the power lever at a rate sufficiently slow enough to avoid these adverse effects. Since permissible rates of increase in engine speed or thrust vary with specific flight conditions, the pilot is required to be further aware of which specific rate is applicable to his present flight condition. Hence, significant pilot attention is required to accomplish a change in engine speed or thrust and consequently the pilot is not fee to attend to other equally important tasks required in the operation of the aircraft.